I really don't see how someone can say that
Godzilla vs. Hedorah is a kids film in any way. It's the most graphic Showa film, outside of
Godzilla, and has some pretty disturbing images, which have already been mentioned.
If anything,
Godzilla vs. Hedorah is the most adult-oriented Godzilla film since
Godzilla.
The subject matter is an adult one, but I've seen adult issues handled in children's movies and shows before. But doesn't mean the movies are serious.
To me, yeah, it does mean they're serious. It means that the filmmakers care about their work so much, that they're willing to take a rising subject that the filmmakers are concerned about, and make a statement about said subject. It means you have a certain respect for your own work when you do something like that. In response, I respect the filmmakers for doing that, thus I take their work seriously.